Certain types of fiber optic systems require couplers in which at least a portion of the light propagating in an optical fiber is coupled to one or more output fibers. The present invention relates to such fiber optic couplers and more particularly to an improved method of making such fiber optic couplers.
It has been known that coupling occurs between two closely spaced cores in a multiple core device. The coupling efficiency increases with decreasing core separation and, in the case of single-mode cores, with decreasing core diameter. Couplers based on these principles are capable of low loss operation, i.e. they exhibit an excess loss of about 1 dB or less.
Reproducible methods of making rugged optical fiber couplers are disclosed in the aforementioned related applications Ser. No. 204,620 and Ser. No. 223,423. In accordance with the methods disclosed in those applications, a plurality of suitably prepared glass fibers, each having a core and cladding, are disposed within the longitudinal aperture of a glass capillary tube, the fibers extending beyond each end thereof. The midregion of the tube is heated and collapsed about the fibers to form a solid midregion, and a portion of the collapsed midregion is stretched to reduce the diameter thereof and to decrease the distance between the fiber cores, whereby optical power propagating in one fiber is coupled to the other.
Other methods employ the steps of twisting and fusing a plurality of fibers, or the steps of grinding or etching cladding from a plurality of fibers and then joining the resultant fibers in such a manner that the mode fields overlap.
Regardless of the process used, all commercial fiber couplers known today have optical fiber leads or "pigtails" extending from both ends thereof so that the coupler can be connected into a system using standard connectors or fusion splices. These pigtails can become tangled and can make transporting couplers a rather cumbersome task. Also, each pigtail must be coupled in the field to other fibers; this adds considerable cost to the implementation of the coupler. Furthermore, due to the lengths of the pigtails, the polarization of an optical signal injected into one pigtail might not be maintained as it propagates through that pigtail, the coupler, and the output pigtail. This effect on the polarization of the input signal is exacerbated when the connector and pigtails are subjected to temperature extremes.